Show progress bar in Blender 2.8 exporter
This is all I've found so far:

import bpy

win = bpy.context.window_manager

win.progress_begin(0, 100)

for i in range(1,100) :
    win.progress_update(i)

win.progress_end()


Update

According to the source code this was intended to be a real progress bar, but that was back in 2013, unfirtunately it seems to have fallen into obscurity.